A variety of techniques and instruments have been developed for use in the removal or repair of tissue in arteries and similar body passageways. A frequent objective of such techniques and instruments is the removal of atherosclerotic plaques in a patient's arteries. Atherosclerosis is characterized by the buildup of fatty deposits (atheromas) in the intimal layer (under the endothelium) of a patient's blood vessels. Very often over time, what initially is deposited as relatively soft cholesterol-rich atheromatous material hardens into a calcified atherosclerotic plaque. Such atheromas restrict the flow of blood, and therefore often are referred to as stenotic lesions or stenoses, the blocking material being referred to as stenotic material. If left untreated, such stenoses can cause angina, hypertension, myocardial infarction, strokes and the like.
Several kinds of atherectomy devices have been developed for attempting to remove some or all of such stenotic material. In one type of device, such as that shown in U.S. Pat. No. 4,990,134 (issued to Auth), a rotating buff covered with an abrasive cutting material such as diamond grit (diamond particles or dust) is carried at the distal end of a flexible drive shaft. The ability of diamond dust covered buffs to remove human soft tissue at high surface speeds (e.g., small diameter buffs rotated at about 200,000 rpm) has been known for some time and has been utilized in dentistry since at least the early 1980's to remove soft gum tissue (see, e.g., "Premier Two Striper.RTM. Gingival Curettage" (Abrasive Technology, Inc. 1982); "Premier Two Striper's Crown & Bridge Techniques" (Abrasive Technology, Inc. 1981); H. Gilmore, et. al, Operative Dentistry (C. V. Mosby Company 1982, 4th ed.), pp. 64-65, 69, 348-350; R. Tupac, et al., "A Comparison of Cord Gingival Displacement With the Gingitage Technique," Journal of Prosthetic (Nov. 1981, pp.509-515); and Premier Presents Two Striper.RTM. Dental Diamond Instruments (Abrasive Technology, Inc. 1989). The buff in the Auth device and in such dental devices is rotated at speeds in the range of 20,000 to 200,000 rpm or more, which, depending on the diameter of the buff, can provide surface speeds of the abrasive particles on the buff in the range of 40 ft/sec. Auth claims that at surface speeds below 40 ft/sec the abrasive buff will remove hardened atherosclerotic material but will not damage normal elastic soft tissue of the vessel wall. Auth also admits that at surface speeds above 40 ft/sec the abrasive burr will remove both hardened and soft tissue. See, e.g., U.S. Pat. No. 4,990,134 at col. 3, lines 20-23.
Unfortunately, not all atherosclerotic plaques are hardened, calcified atherosclerotic plaques. Moreover, the mechanical properties of the soft plaques are very often quite close to the mechanical properties of the soft wail of the vessel. Thus, one cannot safely rely entirely on the differential cutting properties of such abrasive burrs to remove atherosclerotic material from an arterial wall, particularly where one is attempting to remove all or almost all of the atherosclerotic material. See, e.g., Atherectomy, A Physicians Guide, (Strategic Business Development, Inc., 1990), pp. 89, 94-96. Furthermore, in clinical practice, the Auth buff is virtually always rotated at speeds of at least about 155,000 rpm. At such speeds a diamond dust covered buff with a diameter of 1.5 mm achieves a surface speed of 40 ft/sec, the very speed at which the differential cutting effect becomes limited, at best (i.e., the buff removes both hard and soft tissue).
Thus, a significant drawback has been recognized in use of the Auth-type of buff. Although under some conditions the differential cutting properties of such buffs are effective to protect healthy tissue, in many circumstances the buff nevertheless can abrade at least a portion of the healthy tissue, creating a risk of perforation. This is particularly true at higher rotational speeds. A majority of atherosclerotic lesions are asymmetrical (i.e., the atherosclerotic plaque is thicker on one side of the artery than on the other). Moreover, pressure of the buff against the atherosclerotic plaque is achieved only by the use of a buff having a diameter slightly larger than the opening through the stenotic passageway. Thus, since the stenotic material will be entirely removed on the thinner side of an eccentric lesion before it will be removed on the other, thicker side of the lesion, during removal of the remaining thicker portion of the atherosclerotic plaque the buff necessarily will be engaging healthy tissue on the side which has been cleared---indeed, lateral pressure by such healthy tissue against the buff is required to keep the buff in contact with the remaining stenotic tissue on the opposite wall of the passageway. Thus, in clinical practice (balancing safety and residual stenosis), physicians typically used an undersized buff and are not able to remove the entire stenosis--e.g., on a patient having a coronary artery with an original diameter estimated to be 3 mm, rarely would a physician use a buff diameter of more than about 2 mm. See, e.g., Atherectomy, A Physicians Guide, (Strategic Business Development, Inc., 1990), p. 96. These risks are enhanced at high rotational speeds where the differential cutting phenomenon is significantly diminished.
Typically, fluoroscopy is utilized to assist the physician in placing the Auth-type buff in the general location of a stenosis in an artery. This imaging technique does not provide cross-sectional imaging of the artery and, thus, significantly limits the ability of the physician to monitor in real-time the actual removal of stenotic tissue. As a result, the physician's ability to thoroughly remove the stenotic lesion is limited. Unfortunately, conventional intravascular ultrasound imaging equipment, which allows cross-sectional imaging of the arteries, cannot be used simultaneously with the Auth-type device for two reasons. First, the Auth buff itself completely occludes the stenotic portion of the artery during the procedure and therefore leaves no room for an intravascular ultrasound catheter to be positioned in the arterial passageway next to the buff. Second, the Auth-type buff is not sonolucent and therefore will not permit ultrasonic imaging from inside of the buff.
In addition, the Auth device has three drawbacks due to the fact that a separately manufactured abrasive buff must be attached to (or near) the distal end of the flexible drive shaft:
(1) First, the connection between the buff and the drive shaft is critical, in that it must be secure against failure. This requirement therefore adds to the cost of producing the device. PA1 (2) Second, the size of the burr, particularly the diameter of the buff, necessarily limits the ability of the device to safely initiate opening of very tight stenotic lesions, particularly those located more distally in branches of major coronary arteries. PA1 (3) Third, since the buff is made from a solid, inflexible metal, when it is used in tortuous arteries the length of the buff must be kept relatively short in order to allow the buff to navigate the bends and curves of the artery. For a buff of a given diameter, the length of the buff defines how rapidly the transition from its smallest diameter (close to the diameter of the drive shaft) to its maximum diameter must occur. A longer burr may have a more gently sloping profile, while a shorter buff must have a steeper profile. Thus, the inflexibility of the Auth-type buff requires the buff to be relatively blunt. PA1 (1) the invention does not require a separately manufactured buff to be attached to the drive shaft; PA1 (2) the overall diameter of the abrasive segment for a given drive shaft can be made significantly smaller than an abrasive buff for the same diameter drive shaft, thereby allowing treatment of extremely tight stenotic lesions, particularly those located more distally in major coronary arteries or branches of such arteries; PA1 (3) the abrasive segment can be made flexible and can be made longer (than an Auth-type buff of the same diameter), thus allowing the abrasive segment to have a very gently sloping profile, permitting, as a result, treatment of even very tortuous arteries; and PA1 (4) the invention permits use of intravascular ultrasound imaging to monitor the removal of stenotic tissue as it is being removed, thus reducing the risk of perforation, particularly at high rotational speeds where the differential cutting phenomenon is significantly reduced.